In an apparatus using a charged-particle beam for electron-beam lithography or ion-beam lithography, an electron beam or ion beam is directed to a target. Sometimes, the position at which the beam strikes the target is shifted to write a desired pattern. In another application, secondary electrons or other radiation produced by the irradiation of the beam is detected to observe the surface of the target. Where the beam is accelerated at a high accelerating voltage of 30 to 50 KV to irradiate the target, the high energy of the beam etches away the surface of the target changing the topographical features of the surface of the target. Therefore, reducing the accelerating voltage may be contemplated to decrease the landing energy. However, lowering the accelerating voltage makes it impossible to finely focus the beam with the condenser lenses. Consequently, quite accurate illumination of the beam cannot be accomplished, though the target is prevented from being etched.
Referring to FIG. 1, there is shown an improved apparatus utilizing an electron beam. This apparatus includes an electron gun 1 comprising a cathode 2, a Wehnelt electrode 3, and an accelerating electrode 4. An accelerating voltage source 5 supplies an accelerating voltage between the accelerating electrode 4 and the cathode 2. The electron beam emitted by the gun 1 is accelerated and then sharply focused onto a target 8 by a condenser lens 6 and an objective lens 7. The position on the target 8 at which the beam falls on it is moved by supplying a deflecting signal to a deflecting coil 9. A retarding field power supply 10 applies a voltage to the target 8 to retard the beam impinging on it.
The accelerating voltage source 5 applies an accelerating voltage, say -30 KV, to the cathode 2. The electron beam enters the condenser lens 6 and the objective lens 7 with relatively high energy. Therefore, the beam is finely focused on the target 8. Since the power supply 10 applies a retarding voltage of -27 KV, for example, to the target 8, a retarding electric field is set up between the objective lens 7 and the target 8 to retard the beam impinging on the target 8. That is, the electron landing voltage is substantially 3 KV.
Thus, in this apparatus, the electron beam enters the condenser lens and the objective lens with high energy and so it can be focused sharply. Then, it strikes the target 8 with low energy, preventing the surface of the target from being etched away. Hence, the target can be continuously irradiated with the beam without varying the shape of the surface of the target.
In the aforementioned configuration, the retarding electric field is produced between the objective lens 7 and the target 8. If the surface of the target 8 is irregular, then the retarding field is disturbed and becomes nonuniform. This makes it difficult to accurately focus the beam. Another problem arises from the fact that the target 8 is exposed to the retarding electric field. If a very weak electric discharge takes place between the target and the objective lens, then the target may be damaged. Where a microchannel plate which is put at ground potential and serves as a secondary electron detector is disposed near the target, secondary electrons emanating from the target are accelerated at a high voltage of 27 KV and hit the detector. For this reason, the detector deteriorates in performance rapidly.
This improved apparatus is disclosed in an article entitled "Retarding field optics for practical electron beam lithography", T. H. Newman and R. F. W. Pease, SPIE Vol. 471, Electron-Beam, X-ray, and Ion-beam Techniques for Submicrometer Lithographies III (1984).
FIG. 2 shows another apparatus producing an ion beam and having an electrostatic retarding field. This apparatus includes an ion gun 14 consisting of an ion emitter 11, an extraction electrode 12, and an accelerating electrode 13. The ion beam generated and accelerated by the gun is sharply focused onto a target 17 by a condenser lens 15 and an objective lens 16. The lenses 15 and 16 each consist of an einzel lens. The condenser lens 15 has an electrode 15a at it center. A voltage source 18 applies a lens voltage to the central electrode 15a. Also, the objective lens 16 is centrally equipped with an electrode l6a on which a lens voltage is supplied from a voltage source 19. The lenses 15 and 16 have outer electrodes 15b and 16b, respectively, which are grounded. Another voltage source 20 imposes a retarding voltage on the target 17.
In this structure, an accelerating voltage source 21 applies an accelerating voltage of 30 KV, for example, to the emitter 11. The ion beam is accelerated at an accelerating voltage of +30 KV and enters the objective lens 16 with high energy. The lens 16 sharply focuses the beam. The target 17 is applied with a retarding voltage of +27 KV from the voltage source 20. As a result, a retarding electric field is produced between the target 17 and the grounded outer electrode 16b of the objective lens 16. The ion beam is retarded and impinges on the target 17. That is, the ion landing voltage is substantially -3 KV.
In this instrument, the ion beam enters the electrostatic lenses with high energy and so it is finely focused. However, the ion landing energy is made low. This prevents the surface of the target from being etched. Consequently, the position on the target 17 at which the sharply focused beam hits the target can be controlled quite accurately without changing the topographical features of the surface of the target.
When the surface of the target is observed with the ion beam, or when a mark on the target is detected in ion-beam lithography, secondary electrons produced as a result of bombardment of the target by the incident beam must be detected. However, in the above-described apparatus, secondary electrons emanating from the target are attracted back to the target 17, because the retarding field is set up between the objective lens 16 and the target 17, thus making the potential at the target 17 higher than the potential at the objective lens 16. Hence, such secondary electrons cannot be detected even if a detector is positioned close to the target.